The present invention relates to an image processing apparatus and method for encoding an image signal, and a storage medium.
When a given type of phosphor is irradiated with radiation (X-rays, xcex1-rays, xcex2-rays, xcex3-rays, electron beam, ultraviolet rays, or the like), it partially stores radiation energy. When that phosphor is then irradiated with excitation light, such as visible light or the like, it emits light by stimulation in accordance with the stored energy. The phosphor that exhibits such nature is called a storage phosphor (photostimulable phosphor). The present applicant has already proposed a radiation image information recording/reproduction system (Japanese Patent Laid-Open Nos. 55-12429, 56-11395, and the like). In this system, using such storage phosphor, radiation image information of an object to be sensed, such as a human body or the like, is temporarily recorded on a storage phosphor sheet, and the storage phosphor sheet is scanned while being irradiated with excitation light, such as a laser beam or the like, so as to emit light by stimulation. The emitted light is photoelectrically read so as to obtain an image signal, and a radiation image of the object is output as a visible image to a recording medium, such as a photosensitive material, or to a display device, such as a CRT or the like, on the basis of the image signal.
Also, an apparatus which senses an X-ray image of an object, by light emitted by stimulation using a semiconductor sensor in the same manner as in the above system, has been developed in recent years. Such apparatus has a practical merit, i.e., it can record an image over a much broader radiation exposure range than a radiation photographic system using conventional silver halide photographs. That is, X-rays over a very broad range are read by photoelectric conversion means so as to be converted into an electrical signal, and a visible image based on a radiation image is output to a recording medium, such as a photosensitive material, or to a display device, such as a CRT or the like, using the electrical signal, thus obtaining a radiation image which is free from any variations of the dose of radiation.
Since such X-ray image contains a very large amount of information, storage or transmission of that image information requires a huge information volume. For this reason, storage and transmission of such image information use high-efficiency coding that reduces the data size by removing redundancy of an image or changing the contents of an image to a degree at which deterioration of image quality is not visually recognizable.
For example, JPEG recommended by ISO and ITU-T as an international standard coding scheme of still image uses DPCM for reversible compression, and discrete cosine transformation (DCT) for irreversible compression. A detailed description of JPEG will be omitted since they are described in ITU-T Recommendation T.81 | ISO/IEC 10918-1 and the like.
In recent years, many studies about compression methods using discrete wavelet transformation (DWT) have been made. One feature of the compression method using DWT is that it is free from any blocking artifact which is observed in DCT.
On the other hand, as for compression of an X-ray image, the following means for efficiently improving the compression ratio may be used. That is, a region of interest (important region) is set, a low compression ratio is set in that region of interest to minimize deterioration of image quality, and the image of the region of interest is preferentially used compared to those of other regions. However, it is not always easy to select a position of an image where the region of interest is set, since it may require some knowledge concerning medical diagnosis.
The present invention has been made in consideration of the above prior art, and has as its object to provide an image processing method and apparatus which can automatically encode a region of interest of an X-ray image, and a storage medium.
It is another object of the present invention to provide an image processing method and apparatus which compress and encode a region of interest of an X-ray image so as to be able to be decoded earlier than other regions, thereby encoding the region of interest at a low compression ratio with high image quality, and a storage medium.
It is still another object of the present invention to provide an image processing method and apparatus which can decode a region of interest of an X-ray image with higher image quality.
In order to attain the above described objects, an image processing apparatus of the present invention for encoding an X-ray image, comprising:
irradiation field detection means for detecting an X-ray irradiation field region of an input X-ray image; through region detection means for detecting a through region in the irradiation field region detected by said irradiation field detection means; region of interest extraction means for extracting a region of interest in the irradiation field region on the basis of the through region detected by said through region detection means; transformation means for transforming the X-ray image using a discrete wavelet transformation; and encoding means for encoding coefficient values corresponding to the region of interest of coefficients obtained by said transformation means after shifting up the coefficient values with respect to coefficients corresponding to the non-region-of-interest.
In order to attain the above described objects, an image processing method of the present invention for encoding an X-ray image, comprising:
an irradiation field detection step of detecting an X-ray irradiation field region of an input X-ray image; a through region detection step of detecting a through region in the X-ray irradiation field region; a region of interest extracting step of extracting a region of interest in the irradiation field region on the basis of the through region; a transformation step of transforming the X-ray image by using a discrete wavelet transformation and obtaining coefficient values; and an encoding step of encoding the coefficient values after shifting up the coefficient values with respect to coefficients corresponding to the non-region-of-interest.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.